c++ - Are there any tricks to use std::cin to initialize a const variable?

Question

Common std::cin usage

int X;
cin >> X;

The main disadvantage of this is that X cannot be const. It can easily introduce bugs; and I am looking for some trick to be able to create a const value, and write to it just once.

The naive solution

// Naive
int X_temp;
cin >> X_temp;
const int X = X_temp;

You could obviously improve it by changing X to const&; still, the original variable can be modified.

I'm looking for a short and clever solution of how to do this. I am sure I am not the only one who will benefit from a good answer to this question.

// EDIT: I'd like the solution to be easily extensible to the other types (let's say, all PODs, std::string and movable-copyable classes with trivial constructor) (if it doesn't make sense, please let me know in comments).

Answer 1

I'd probably opt for returning an optional, since the streaming could fail. To test if it did (in case you want to assign another value), use get_value_or(default), as shown in the example.

template<class T, class Stream>
boost::optional<T> stream_get(Stream& s){
  T x;
  if(s >> x)
    return std::move(x); // automatic move doesn't happen since
                         // return type is different from T
  return boost::none;
}

Live example.

To further ensure that the user gets no wall-of-overloads presented when T is not input-streamable, you can write a trait class that checks if stream >> T_lvalue is valid and static_assert if it's not:

namespace detail{
template<class T, class Stream>
struct is_input_streamable_test{
  template<class U>
  static auto f(U* u, Stream* s = 0) -> decltype((*s >> *u), int());
  template<class>
  static void f(...);

  static constexpr bool value = !std::is_void<decltype(f<T>(0))>::value;
};

template<class T, class Stream>
struct is_input_streamable
  : std::integral_constant<bool, is_input_streamable_test<T, Stream>::value>
{
};

template<class T, class Stream>
bool do_stream(T& v, Stream& s){ return s >> v; }
} // detail::

template<class T, class Stream>
boost::optional<T> stream_get(Stream& s){
  using iis = detail::is_input_streamable<T, Stream>;
  static_assert(iis::value, "T must support 'stream >> value_of_T'");
  T x;
  if(detail::do_stream(x, s))
    return std::move(x); // automatic move doesn't happen since
                         // return type is different from T
  return boost::none;
}

Live example.

I'm using a detail::do_stream function, since otherwise s >> x would still be parsed inside get_stream and you'd still get the wall-of-overloads that we wanted to avoid when the static_assert fires. Delegating this operation to a different function makes this work.